Method for fabricating a thin film bulk acoustic wave resonator (FBAR) on a glass substrate

ABSTRACT

A method for fabricating a Thin Film Bulk Acoustic Wave Resonator (FBAR). The method comprises the steps of: (A) forming a sacrificial layer comprising one of a metal and a polymer over a selected portion of a substrate; (B) forming a protective layer on the sacrificial layer and on selected portions of the substrate; (C) forming a bottom electrode layer on a selected portion of the protective layer; (D) forming a piezoelectric layer on a selected portion of the bottom electrode layer and on a selected portion of the protective layer; (E) forming a top electrode on a selected portion of the piezoelectric layer; and (F) removing the sacrificial layer to form an air gap. The use of a metal or a polymer material to form sacrificial layers has several advantages over the use of zinc-oxide (ZnO) to form such layers. In accordance with a further aspect of the invention, an FBAR is provided which includes a glass substrate. The use of glass to form substrates offers several advantages over the use of other materials to form substrates. By example, most types of glass are less expensive than semiconductor materials, and exhibit low permittivity characteristics, and low parasitic capacitances. In addition, most glass materials are substantially loss free when being used in microwave frequency applications.

FIELD OF THE INVENTION

[0001] The invention relates to resonators and, in particular, theinvention relates to a method for fabricating Thin Film Bulk AcousticWave Resonators (FBARs).

BACKGROUND OF THE INVENTION

[0002] It is known to construct Thin Film Bulk Acoustic Wave Resonators(FBARs) on semiconductor wafers including those which are comprised ofSilicon (Si) or gallium arsenide (GaAs). For example, in an articleentitled “Acoustic Bulk Wave Composite Resonators”, Applied PhysicsLett., Vol. 38, No. 3, pp. 125-127, Feb. 1, 1981, by K. M. Lakin and J.S. Wang, an acoustic bulk wave resonator is disclosed which comprises athin film piezoelectric layer of Zinc-Oxide (ZnO) sputtered over a thinmembrane of Silicon (Si).

[0003] Unfortunately, semiconductor materials have high conductivitiesand high dielectric permittivity characteristics. These characteristicscan have a deleterious effect on piezoelectric coupling efficiency andresonator quality factors, as is described in an article entitled“Temperature Compensated High Coupling and High Quality Factor ZnO/SiO₂Bulk Wave Resonators on High Resistance Substrates”, IEEE UltrasonicsSymp., 1984, pp. 405-410, by T. Shiosaki, T. Fukuichi, M. Tokuda, and A.Kawabata.

[0004] It is known to reduce ohmic losses exhibited by semiconductorwafers by using semi-insulating semiconductor wafers, as is evidenced byan article entitled “Coplanar Waveguides and Microwave Inductors onSilicon Substrates”, IEEE Trans. Microwave Theory Tech., vol. 43, no. 9,pp. 2016-2021, 1995, by Adolfo C. Reyes, Samir M. El-Ghazaly, Steve J.Dorn, Michael Dydyk, Dieter K. Schroder, and Howard Patterson. However,the use of these types of wafers necessitates the use of expensivespecial grade materials, and does not eliminate the presence of straycapacitances.

[0005] Additionally, semiconductor wafers and crystalline wafers need tobe carefully polished after being cut from a crystal in order to smooththeir surfaces. The polishing process can be expensive.

[0006] It would be advantageous to provide a substrate that is formed ofa low cost material and which exhibits a low permittivity characteristicand low parasitic capacitances. It would also be advantageous to providea substrate that is formed of a material that does not need to bepolished in order to smooth its surfaces.

[0007] It is known to construct so called “bridge” structures on FBARsubstrate surfaces using a sacrificial layer that is formed ofzinc-oxide (ZnO), as is evidenced by an article entitled “An Air-GapType Piezoelectric Composite Thin Film Resonator”, IEEE Proc. 39thAnnual Symp. Freq. Control, pp. 361-366, 1985, by Hiroaki Satoh, YasuoEbata, Hitoshi Suzuki, and Choji Narahara. Similarly, in an articleentitled “Multi-layered Ultrasonic Transducers Employing Air-GapStructure”, IEEE Trans. Ultrason. Ferroelec. Freq. Control, vol. 42, no.3, May 1995, by Susumu Yoshimoto, Masamichi Sakamoto, Ken-ya Hashimoto,and Masatsune Yamaguchi, a multi-layered ultrasonic transducer isdisclosed which includes an air gap formed by the removal of a“sacrificial” ZnO layer.

[0008] During the fabrication of these types of FBARs, a sacrificiallayer of ZnO are deposited (e.g., sputtered) over a substrate. Thesacrificial layer is later removed via an etching step that is performedafter all of the layers of the FBAR have been completely formed. Onedrawback of this process is that the steps of sputtering and forming thesacrificial layer can be tedious and time-consuming. This is because ZnOis a ceramic material, and thus is brittle and has a low thermalconductivity. If, by example, very high power is employed during thesputtering of the ZnO, the “target” substrate may break. Also, thegrowth rate of the ZnO must be controlled to produce a correct crystalorientation and crystallite size distribution. Thus, the growth rate mayneed to be limited to only 2 μm/h.

[0009] Another drawback of using a sacrificial layer formed of ZnO isthat the surface of the crystalline ZnO film is textured, and thuscauses acoustic energy scattering losses to occur within the FBAR. Also,the textured surface of the ZnO may cause the surface of a layer (e.g.,the bridge layer) formed over the sacrificial ZnO layer to becomedeformed. A further drawback of employing a sacrificial layer of ZnO isthat during the etching of the layer to form an air gap, a piezoelectricZnO layer that is formed on the sacrificial layer can become damaged.

[0010] In view of these problems, it can be appreciated that it would beadvantageous to provide a method for fabricating an FBAR using asacrificial layer that is formed of a material having characteristicsthat are more beneficial than those of ZnO and other materialsconventionally used to form sacrificial layers.

OBJECTS OF THE INVENTION

[0011] It is a first object of this invention to provide a Thin FilmBulk Acoustic Wave Resonator (FBAR) having a sacrificial layer that isformed of a material that has more beneficial characteristics thanconventional materials that are used to form sacrificial layers.

[0012] It is a second object of this invention to provide an improvedmethod for fabricating a Thin Film Bulk Acoustic Wave Resonator (FBAR).

[0013] It is a third object of this invention to provide a Thin FilmBulk Acoustic Wave Resonator (FBAR) having a substrate that is formed ofa material that has more beneficial characteristics than conventionalmaterials that are used to form substrates.

[0014] Further objects and advantages of this invention will becomeapparent from a consideration of the drawings and ensuing description.

SUMMARY OF THE INVENTION

[0015] The foregoing and other problems are overcome and the objects ofthe invention are realized by a method for fabricating a Thin Film BulkAcoustic Wave Resonator (FBAR). The method of the present inventionemploys a sacrificial layer that is comprised of either a metal orpolymer material instead of a conventionally-used material such as, byexample, zinc-oxide (ZnO). The use of these materials to formsacrificial layers has many advantages over the use of ZnO to form theselayers.

[0016] In accordance with the method of the invention, a first stepincludes sputtering a metal such as, be example, copper (Cu) over asubstrate. The sputtered Cu is then patterned to form a sacrificiallayer.

[0017] A next step includes depositing silicon dioxide (SiO₂) over thesacrificial layer and over selected portions of the substrate to form afirst SiO₂ layer (also referred to as a “first protective layer”). Thislayer may not be needed in cases in which the piezoelectric layercomprises a material which will not be detrimentally affected by theetching of the sacrificial layer (which will be described below).Thereafter, a metallic material such as, by example, gold (Au) isdeposited over a selected portion of the first SiO₂ layer. The depositedgold is then patterned to form a bottom electrode layer.

[0018] A next step includes depositing zinc-oxide (ZnO) over a selectedportion of the bottom electrode layer, and over a selected portion ofthe first SiO₂ layer. The ZnO is then patterned to form a ZnO layer(also referred to as a “piezoelectric layer”).

[0019] A next step of the process includes depositing a further metallicmaterial such as, by example, gold, over a selected portion of the ZnOlayer. Thereafter, the deposited gold is patterned to form a topelectrode layer. In appropriate cases, a second protective layer mayalso be formed on the structure to protect the piezoelectric layerduring the formation of the vias and/or during the etching of thesacrificial layer which will be described below.

[0020] Vias are then formed in the structure so that the sacrificiallayer can be removed. Thereafter, the sacrificial layer is removedthrough the vias to form an air gap.

[0021] The fabrication process of the invention may also be performedusing a sacrificial layer that is comprised of a polymer instead of ametal. The steps of this process are similar to those described above,except that the steps of depositing, patterning, and removing thesacrificial layer (e.g., the polymer) are performed in a differentmanner. By example, the step of depositing the polymer is performed byspinning the polymer onto the substrate. The polymer is then patternedto form the sacrificial layer. After each of the other layers and thevias of the FBAR have been formed in the same manner as described above,the sacrificial layer is removed to form the air gap.

[0022] The type of polymer used to form the sacrificial layer ispreferably one that can withstand the high temperatures that can bereached during the sputtering of the ZnO layer.

[0023] In accordance with another aspect of the invention, the methodfor fabricating FBARs may be performed by removing the sacrificial layerprior to the deposition of the material forming the ZnO layer. For thismethod, the same steps described above are performed, except that afterthe first SiO₂layer and the bottom electrode layer have been formed, thesacrificial layer is then removed to form the air gap in the same manneras described above. Thereafter, the steps of forming the ZnO layer andthe top electrode layer are carried out in the same manner as describedabove.

[0024] The use of metals and polymers to form sacrificial layers hasmany advantages over the use of most conventionally-used materialsincluding, by example, ZnO. For example, FBARs having sacrificial layersthat are comprised of metals or polymers can he fabricated more quicklythan the FBARs having sacrificial layers formed of ZnO. Also, metals andpolymers generally have smoother surfaces than ZnO. Moreover, metals andpolymers can be etched using chemicals that are not harmful topiezoelectric layers formed of, by example, ZnO.

[0025] In accordance with a further aspect of the invention, an FBAR isprovided which includes a glass substrate. The FBAR is formed of similarlayers as the FBARs described above, and may be fabricated in accordancewith the method of the invention. The use of glass to form substratesoffers several advantages over the use of other materials to formsubstrates. Glass is inexpensive and thus can be used to form substrateshaving large surface areas at an inexpensive cost. Also, most glassmaterials exhibit low permittivity characteristics and low parasiticcapacitances. Moreover, most glass materials become substantially lossfree when used in microwave frequency applications.

[0026] A further advantage of using glass to form substrates is that,unlike semiconductor materials, glass can have inherently smoothsurfaces and thus requires little or no polishing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The above set forth and other features of the invention are mademore apparent in the ensuing Detailed Description of the Invention whenread in conjunction with the attached Drawings, wherein:

[0028]FIG. 1a illustrates a cross section of an exemplary FBAR having aglass substrate.

[0029]FIG. 1b illustrates a table showing permittivities for varioustypes of materials.

[0030]FIG. 2a illustrates a top view of a portion of an exemplary FBARhaving vias.

[0031]FIG. 2b illustrates a cross section of a portion of an exemplaryFBAR.

[0032]FIG. 3a illustrates a cross section of a portion of a Thin FilmBulk Acoustic Wave Resonator (FBAR) that is formed in accordance with astep of a fabrication process of the invention, for a case in which theportion of the FBAR comprises a sacrificial layer formed of Copper (Cu).

[0033]FIG. 3b illustrates a cross section of the FBAR FIG. 3a as itappears after a further step of the fabrication process performed inaccordance with the invention.

[0034]FIG. 4 illustrates a cross section of the FBAR of FIG. 3b as itappears after a further step of the fabrication process performed inaccordance with the invention.

[0035]FIG. 5 illustrates a cross section of the FBAR resonator of FIG. 4as it appears after a further step of the fabrication process performedin accordance with the invention.

[0036]FIG. 6a illustrates a cross section of a portion of Thin Film BulkAcoustic Wave Resonator (FBAR) that is formed in accordance with a stepof the fabrication process of the invention, for a case in which theportion of the FBAR comprises a sacrificial layer formed of a polymermaterial.

[0037]FIG. 6b illustrates a cross section of the FEAR FIG. 6a as itappears after a further step of the fabrication process performed inaccordance with the invention.

[0038]FIG. 7 illustrates a cross section of the FBAR of FIG. 6b as itappears after a further step of the fabrication process performed inaccordance with the invention.

[0039]FIG. 8 illustrates a cross section of the FBAR of FIG. 7 as itappears after a further step of the fabrication process performed inaccordance with the invention.

[0040]FIG. 9a illustrates a cross section of a portion of a Thin FilmBulk Acoustic Wave Resonator (FBAR) that is formed in accordance with astep of an alternate fabrication process of the invention.

[0041]FIG. 9b illustrates a cross section of the FBAR FIG. 9a as itappears after a further step of the alternate fabrication process of theinvention.

[0042]FIG. 10 illustrates a cross section of the FBAR of FIG. 9b as itappears after a further step of the alternate fabrication process of theinvention.

[0043]FIG. 11 illustrates a cross section of the FBAR resonator of FIG.10 as it appears after a further step of the alternate fabricationprocess of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0044] The inventors have developed an improved method for fabricatingThin Film Bulk Acoustic Wave Resonators (FBARs) having air gaps. Unlikemost of the conventional methods for fabricating FBARs, which employsacrificial layers formed of, by example, zinc-oxide (ZnO), the methodof the present invention employs a sacrificial layer that is comprisedof either a metal or a polymer material. The use of these materials toform sacrificial layers has many advantages over the use of ZnO to formthese layers, as will be described below.

[0045] The method of fabricating FBARs in accordance with the inventioncan be understood in view of FIGS. 3a-5. As a first step of thefabrication process, a metal such as, by example, copper (Cu) issputtered or otherwise deposited over a substrate 42. It is assumed thatthe substrate is comprised of glass, in accordance with an aspect ofthis invention which will be further described below, although any othersuitable solid material may be employed. The sputtered Cu is thenpatterned so that it has a thickness of, by example, approximately 2000nm, thereby forming bridges.

[0046] Thereafter, the Cu is wet etched to form tapered edges 44 a and44 b. In this manner, a sacrificial layer (also referred to as a “baselayer”) 44 is formed.

[0047] It should be noted that other metals may be used in lieu ofcopper to form the sacrificial layer 44. By example, the sacrificiallayer 44 may be formed of aluminum, zinc, antimony, and any of theelements in the periodic table extending from titanium to copper, fromyttrium to silver, and from lanthanum to gold. Ideally, the metals usedto form the layer 44 are low cost and can be etched without greatdifficulty.

[0048] Referring to FIG. 3b, a next step includes depositingsilicon-dioxide (SiO₂) over the sacrificial layer 44 and over selectedportions of the substrate 42 to form a first SiO₂ layer (also referredto as a “first protective layer”) 48 having a thickness of, by example,approximately 300 nm. Thereafter, a metallic material such as, byexample, gold (Au) is deposited over the a selected portion of the firstSiO₂ layer 48. The deposited gold is then patterned to form a bottomelectrode layer 50 having a thickness of, by example, approximately 300nm. Other suitable electrically conductive metallic and non-metallicmaterials call also be employed.

[0049] It should be noted that for cases in which the material used toform the piezoelectric layer (to be described below) is notdetrimentally effected by the etching of the sacrificial layer (theetching of the sacrificial layer will be described below), it is notnecessary to employ the first protective layer 48, and the bottomelectrode layer may be formed directly over the sacrificial layer 44 andselected portions of the substrate 42.

[0050] Referring to FIG. 4, a next step includes depositing zinc-oxide(ZnO) over a selected portion of the bottom electrode layer 50, and overa selected portion of the first SiO₂ layer 48. The ZnO may be depositedby, for example, sputtering from a target in a mixture of Argon (Ar) andO₂. After the ZnO is deposited, it is patterned to form a ZnO layer(also referred to as a “piezoelectric layer”) 52 having a selectedthickness (e.g., approximately 2000 nm) that is a function of a desiredresonant frequency of the FBAR.

[0051] A next step of the process includes depositing a further metallicmaterial such as, by example, gold, over a selected portion of the ZnOlayer 52. Thereafter, the deposited gold is patterned to form a topelectrode layer 54, which is illustrated in FIG. 5. As with the firstelectrode layer 50, other suitable electrically conductive metallic andnon-metallic materials can also be employed.

[0052] Vias are then formed in the structure so that the sacrificiallayer 44 can be removed. The vias may be formed in any suitable manner.For example, and referring to FIG. 2a, selected portions of the firstSiO2 layer 48 may be removed to provide openings 47 and 49 for accessingthe sacrificial layer 44 (tapered edges 44 a and 44 b are shown). Itshould be noted that in cases where it is appropriate to protect thepiezoelectric layer 52 during the creation of the vias and/or theremoval of the sacrificial layer, a second layer of SiO₂(also referredto as a “second protective layer”) (not illustrated in FIG. 2a) may beemployed. By example, SiO₂ may be deposited and patterned on a selectedportion of the piezoelectric layer 52 after the piezoelectric layer 52has been formed and before the top electrode layer 54 has been formed.The SiO₂ is then patterned to form a contact area on a top portion ofthe ZnO layer 52. Within the contact area the top electrode layer 54 isthen formed. In other cases where it is appropriate, the secondprotective layer may instead be formed after the formation of the topelectrode layer 54 on the piezoelectric layer 52.

[0053] Vias may also be formed in the structure according to the exampleshown in FIG. 2b. For this example, holes may be formed in the ZnO layer52 by an using an acid etchant. Selected portions of the FBAR and thesurrounding wafer may then be covered by depositing a second protectivelayer of SiO₂ 51 via, for example, a plasma-enhanced chemical vapordeposition (CVD). The SiO₂ is then patterned in fluorine plasma (Fplasma) to form a contact area on a top portion of the ZnO layer 52.Then, the top electrode layer 54 is formed over the contact area andover portions of the second layer of SiO₂ 51. Thereafter, selectedportions of the second layer of SiO₂ 51 and selected portions of thefirst SiO2 layer 48 are patterned to form vias (via 49′ is shown in FIG.2b). For this example, it should be noted that as described above withrespect to FIG. 2a, instead of depositing and patterning the secondprotective layer 51 prior to the formation of the top electrode layer54, the second protective layer 51 may be deposited and patterned afterthe formation of the top electrode layer 54.

[0054] A next step of the fabrication process includes removing thesacrificial layer 44 through the vias by wet etching to form an air gap62. The etching may be performed by using, for example, acids, alkalis,or redox reactions (e.g., ferric chloride). Either selective ornon-selective etching may be used.

[0055] In order to determine whether the device is correctly tuned, theelectrical performance characteristics of the device can then bemeasured in a suitable manner and compared with a model of the device.Any suitable technique may be employed for modeling the device includingthat disclosed in an article entitled “Systematic Design ofStacked-Crystal Filters by Microwave Network Methods”, IEEE Trans.Microwave Theory Tech., vol. MTT-22, pp. 14-25, January 1974, by ArthurBallato, Henry L. Bertoni, and Theodor Tamir.

[0056]FIGS. 6a-8 illustrate steps of the fabrication process of theinvention using a sacrificial layer that is formed of a polymer insteadof a metal. The steps of this process are similar to those describedabove, except that the steps of depositing, patterning, and removing thesacrificial layer (e.g., the polymer) are performed in a differentmanner. Referring to FIG. 6a, by example, the step of depositing thepolymer is preferably performed by spinning the polymer onto thesubstrate 42 thereby providing an inherently smooth surface for furtherprocessing. The polymer is then patterned so that it has a thickness of,by example, approximately 1000 nm, thereby forming bridges. The polymeris also etched to form tapered edges 44 a and 44 b in a similar manneras described above. In this manner, sacrificial layer 60 is formed.After each of the other layers and the vias of the FBAR have been formedin the same manner as described above (FIGS. 6b and 7), the sacrificiallayer 60 is then removed by, for example, etching or plasma ashing. Inthis manner, air gap 62 is formed, which is illustrated in FIG. 8.

[0057] For cases in which organic polymers are used to form thesacrificial layer 60, the sacrificial layer 60 can be etched in, byexample, a plasma containing oxygen. For cases in which siliconepolymers are used, for example, an addition of fluorine may benecessary.

[0058] Also, some types of polymers can be dissolved in organic solvents(e.g., acetone). Unlike the corrosive chemicals that are used for, byexample, anisotropic etching of silicon, organic solvents do not attackZnO. Thus, polymers which can be dissolved by organic solvents arepreferred. Also, the use of this type of bridge structure avoidsdrawbacks that can associated with the performance of, by example, deepanisotropic etching of silicon, which requires the etching of a largesurface area because the etch stopping crystal plane is angled at, byexample, only 54.74 degrees with-respect to the surface of the wafer.

[0059] The type of polymer used to form the sacrificial layer 60 is alsopreferably one that can withstand the high temperatures that can bereached during the sputtering of the ZnO layer 52. The polymer may be,by example, polytetrafluoroethylene or a derivative thereof,polyphenylene sulfide, polyetheretherketone, poly(para phenylenebenzobismidazole), poly(para phenylene benzobisoxazole), poly (paraphenylene benzobismidazole), poly (para phenylene benzobisthiazole), apolyimide, polyimide siloxane, vinyle ethers, polyphenyl, parylene-n,parylene-f, benzocyclobutene.

[0060] In accordance with another aspect of the invention, the methodfor fabricating FBARs may be performed by removing the sacrificial layerprior to the deposition of the material forming the ZnO layer 52. Thismay be understood in view of FIGS. 9a-11 wherein portions of anexemplary FBAR that is formed in accordance with this method of theinvention are illustrated. The FBAR includes a sacrificial layer 44formed of, by example, copper (Cu). For this method, the same stepsdescribed above are performed, except that after the first SiO₂ layer48, the bottom electrode layer 50, and the vias have been formed, thesacrificial layer 44 is then removed to form the air gap 62 in the samemanner as described above. Thereafter, the steps of forming the ZnOlayer 52 and the top electrode layer 54 are carried out in the samemanner as described above.

[0061] The use of metal or polymer materials to form sacrificial layersenables the fabrication process to be performed more quickly than thefabrication processes employing sacrificial layers that are formed of,by example ZnO. This is because ZnO is a brittle ceramic material andthus requires more time to be deposited than metals or polymers whichare not as brittle as ZnO.

[0062] The use of polymers or metals to form sacrificial layers alsoavoids problems that can be associated with the use of mostconventionally-used materials (e.g., zinc-oxide) to form such layers. Byexample, unlike some materials, metals have small grain sizes, and thushave naturally smooth surfaces. Also, polymers can be spun-on during thefabrication of FBAR structures, and develop smooth surfaces duringbaking while the polymer reflows in a liquid state. As a result, thelayer formed over the polymer sacrificial layer does not experiencesurface deformations that are as serious as those which can occur tolayers deposited over most conventionally-used materials (e.g., ZnO).

[0063] Another advantage of using metals or polymers to form sacrificiallayers instead of, by example, ZnO is that metals and polymers can beetched using chemicals that are not harmful to the ZnO layer 52.

[0064] In accordance with a further aspect of the invention, an FBAR isprovided which includes a glass substrate 42, as is illustrated in FIG.1a. The FBAR is formed of similar layers as the FBARs described above,and may be fabricated in accordance with the method of the invention.

[0065] The use of glass to form substrates offers several advantagesover the use of semiconductor materials to form substrates. Oneadvantage is that glass is inexpensive. Thus, glass can be used to formchips having large surface areas at an inexpensive cost, thus providingmore surface area for bonding. Active components such as, by example,transistors and integrated circuits (ICs) can be added onto the chipsby, for example, flip-chip technology.

[0066] Also, most glass materials including, by example, silicate glass,have low permittivity characteristics, and hence exhibit low parasiticcapacitances. Thus, unlike semiconductor materials, which normally havehigh conductivities and high dielectric permittivity characteristics,glass substrates do not exhibit detrimental characteristics such as, byexample, low piezoelectric coupling efficiencies and low resonatorquality factors. FIG. 1b illustrates a table showing the respectivepermittivities of various types of materials.

[0067] In addition, most glass materials except, by example, soda-limeglass, are substantially loss free when being used in microwavefrequency applications.

[0068] A further advantage of using glass to form substrates is that,unlike semiconductor materials, most glasses have naturally smoothsurfaces. Thus, little or no polishing is required to smooth thesurfaces of glass substrates as is required to smooth the surfaces ofsubstrates formed of semiconductor materials.

[0069] Moreover, the thermal expansion characteristics of most types ofglass materials are more similar to those of the other materials formingthe FBAR layers than are the thermal expansion characteristics of, byexample, silicon. As such, in a bonding application, a component may beencapsulated in glass.

[0070] It should be noted that the types of technologies that areavailable for micromachining glass substrates are not as diverse asthose available for micromachining substrates formed of crystallinesemiconductor materials.

[0071] While the invention has been particularly shown and describedwith respect to preferred embodiments thereof, it will be understood bythose skilled in the art that changes in form and details may be madetherein without departing from the scope and spirit of the invention.

What is claimed is:
 1. A method for fabricating a Thin Film BulkAcoustic Wave Resonator (FBAR), comprising the steps of: STEP A: forminga sacrificial layer on a selected portion of a substrate, wherein thesacrificial comprises one of a metal and a polymer; STEP B: forming abottom electrode layer on the sacrificial layer and on selected portionsof the substrate; STEP C: forming a piezoelectric layer on a selectedportion of the bottom electrode layer and on a selected portion of thesubstrate; STEP D: forming a top electrode layer on a selected portionof the piezoelectric layer; STEP E: removing the sacrificial layer toform an air gap beneath at least a portion of the bottom electrodelayer.
 2. A method as set forth in claim 1, wherein STEP A is performedby the steps of: STEP 1: depositing one of the metal and the polymerover the substrate; and STEP 2: patterning the deposited one of themetal and the polymer to form the sacrificial layer.
 3. A method as setforth in claim 2, wherein STEP E is performed by etching the sacrificiallayer using a chemical that is not harmful to the piezoelectric layer.4. A method as set forth in claim 1, wherein STEP A is performed by thesteps of: STEP 1: depositing the metal over the substrate; and STEP 2:patterning the deposited metal to form the sacrificial layer.
 5. Amethod as set forth in claim 4, wherein the sacrificial layer has athickness of about 2000 nm.
 6. A method as set forth in claim 4, whereinthe metal is copper (Cu).
 7. A method as set forth in claim 1, whereinSTEP A is performed by the steps of: STEP 1: spinning the polymer ontothe substrate; and STEP 2: patterning the polymer to form thesacrificial layer.
 8. A method as set forth in claim 7, wherein thesacrificial layer has a thickness of about 1000 nm.
 9. A method as setforth in claim 1, wherein between the performances of STEPs D and E, astep is performed of: forming at least one via through at least one ofthe layers formed in STEPS B-D so that the sacrificial layer can beremoved through the at least one via.
 10. A method as set forth in claim1, wherein the protective layer comprises SiO₂ having a thickness ofabout 300 nm.
 11. A method as set forth in claim 1, wherein the bottomelectrode layer comprises an electrically conductive metal having athickness of about 300 nm.
 12. A method as set forth in claim 1, whereinthe piezoelectric layer comprises zinc-oxide (ZnO) having a thickness ofabout 2000 nm.
 13. A method as set forth in claim 1, wherein the topelectrode layer is comprised of an electrically conductive metal havinga thickness of about 300 nm.
 14. A method as set forth in claim 1,wherein the sacrificial layer is formed of the metal, and wherein STEP Eis performed by wet etching the sacrificial layer.
 15. A method as setforth in claim 1, wherein the sacrificial layer is formed of thepolymer, and wherein STEP E is performed by one of the steps of etchingthe sacrificial layer and plasma ashing the sacrificial layer.
 16. Amethod as set forth in claim 1, wherein the polymer material can survivethe performance of STEP C at an elevated temperature.
 17. A method asset forth in claim 1, wherein the substrate is comprised of a solidmaterial.
 18. A method as set forth in claim 1, wherein STEP E isperformed between the performances of STEP B and STEP C.
 19. A method asset forth in claim 1, wherein the piezoelectric layer compriseszinc-oxide (ZnO) having a thickness that is a function of a desiredresonant frequency of the FBAR.
 20. A method as set forth in claim 1,wherein between the substrate is comprised of a glass.
 21. A method forfabricating a Thin Film Bulk Acoustic Wave Resonator (FBAR), comprisingthe steps of: STEP A: forming a sacrificial layer on a selected portionof a substrate, wherein the sacrificial comprises one of a metal and apolymer; STEP B: forming a first protective layer on the sacrificiallayer and on selected portions of the substrate; STEP C: forming abottom electrode layer on a selected portion of the first protectivelayer; STEP D: forming a piezoelectric layer on a selected portion ofthe bottom electrode layer and on a selected portion of the firstprotective layer; STEP E: forming a top electrode layer on a selectedportion of the piezoelectric layer; STEP F: removing the sacrificiallayer to form an air gap beneath at least a portion of the bottomelectrode layer.
 22. A method as set forth in claim 21, wherein betweenthe performances of STEPs D and E, a further step is performed offorming a second protective layer over selected portions of thepiezoelectric layer so that a portion of the second protective layer anda top portion of the piezoelectric layer form a contact area, andwherein STEP E is performed by forming the top electrode layer withinthe contact area.
 23. A method as set forth in claim 21, wherein betweenthe performances of STEPs E and F, a further step is performed offorming a second protective layer over selected portions of thepiezoelectric layer.
 24. A Thin Film Bulk Acoustic Wave Resonator (FBAR)comprising: a substrate comprised of glass; a piezoelectric layer; abottom electrode layer, at least a portion of said bottom electrodelayer being located between a selected portion of said substrate and aselected portion of said piezoelectric layer; a top electrode layerformed on a top portion of said piezoelectric layer.
 25. A Thin FilmBulk Acoustic Wave Resonator (FBAR) as set forth in claim 24, furthercomprising: a protective layer formed over portions of said substrate;wherein said piezoelectric layer is formed atop a second selectedportion of said protective layer and atop said portion of said bottomelectrode layer; and wherein a first selected portion of said substrateand a first selected portion of said protective layer define an air gaptherebetween.
 26. A Thin Film Bulk Acoustic Wave Resonator (FBAR) as setforth in claim 25, wherein said air gap is formed by a removal of asacrificial layer comprised of one of a polymer and a metal.